1. Field of the Invention
The present invention relates to a semiconductor integrated circuit device including a semiconductor one-time programmable memory using an electrically programmable irreversible storage element and, more particularly, to a program circuit of a semiconductor one-time programmable memory.
2. Description of the Related Art
In the recent semiconductor integrated circuit devices, a nonvolatile one-time programmable (OTP) memory in which stored data does not disappear even when power supply stops is an essential element. The OTP memory is widely used in, e.g., redundancy of a large-capacity memory such as a DRAM or SRAM, tuning of an analog circuit, storage of a code such as an encryption key, and a chip ID for storing management information such as a log in a fabrication process.
For example, a ROM using a laser fuse which is the most inexpensive nonvolatile memory is used for memory redundancy. The laser fuse stores information by blowing upon being irradiated with a laser beam, thereby causing an irreversible change. When the laser fuse ROM is used, however, a fuse blow equipment for programming and a blow step using the same are necessary. In addition, since the minimum dimension of the laser fuse is determined by the wavelength of the laser beam used, downsizing of the fuse cannot keep step with that of other semiconductor devices. Therefore, as downsizing of the other semiconductor devices advances, the ratio of the area occupied by the laser fuse in the chip gradually increases.
Furthermore, since programming of the laser fuse ROM requires irradiation with the laser beam as described above, programming can be performed only in a wafer state. This makes it impossible to perform, e.g., remedy of defects by a high-speed test after packaging, or built-in self repair using a built-in test circuit of a chip. Accordingly, it is being required an electrically programmable nonvolatile memory even in a system using the laser fuse.
On the other hand, in a system formed by using a plurality of chips, various pieces of information can be stored in independent EEPROM chips. However, in an SoC (System on Chip) in which the system is integrated on one chip, a nonvolatile memory must be formed inside the chip. Therefore, embedding a nonvolatile memory which stores electric charge in the floating gate requires additional masks and fabrication processes, and increases the fabrication cost.
Generally, not all information stored in a nonvolatile memory such as redundancy information of the memory need be multi-time Programmable. Accordingly, the OTP (One-Time Programmable) memory which can be embedded by the presently standard CMOS process presumably has a wide demand.
Storage elements which are used in the OTP memory and store information by irreversibly changing the element characteristics will be generally referred to as fuse elements hereinafter. Also, of the fuse elements, those which electrically irreversibly change the element characteristics will be generally referred to as e-fuses (electrical fuses) hereinafter.
Examples of the e-fuse are a poly e-fuse or metal e-fuse which changes the resistance value by supplying a large electric current to a line which is made of polysilicon or a metal in order to intentionally increase the current density, and a gate oxide e-fuse (Gate-Ox eFuse) which causes dielectric breakdown by applying a high voltage to the gate oxide of a MOS transistor, and uses the decrease in resistance caused by the formation of a conduction spot.
For example, a cell of the OTP memory using the gate oxide e-fuse described above is formed as shown in FIG. 5A of Hiroshi Ito et al., “Pure CMOS One-Time Programmable Memory using Gate-Ox Anti-fuse”, Proceedings of the IEEE 2004 CUSTOM INTEGRATED CIRCUITS CONFERENCE, pp. 469-472. In this reference, the cell is formed by using a fabrication process which supports a MOS transistor including gate oxide films having two or more types of thicknesses. A P-channel MOS transistor MPO as the e-fuse element has a thin oxide film, and other MOS transistors have thick oxide films. An N-channel MOS transistor MN0 controls a gate voltage VBT to an appropriate level, thereby limiting the voltage of a node n0 to “VBT-Vth” (Vth is the threshold voltage of the MOS transistor MN0), and preventing the application of a high voltage to a MOS transistor connected to the node n0. This MOS transistor will be referred to as a barrier transistor hereinafter.
Assume that in a programmable state, a terminal VBP is at a high voltage for programming, a terminal VBT is at an appropriate voltage between a VDD level and VBP level, and a terminal PRGp is at a GND level. To program the e-fuse, i.e., to breakdown the gate oxide of the MOS transistor MP0, the terminal PRGp is raised from the GND to a power supply voltage VDD, thereby turning on a MOS transistor MN1, and lowering the node n0 and a node n1 to the GND level. Consequently, the high voltage VBP is applied to the gate oxide of the MOS transistor MP0 to cause breakdown within a short time.
A Joule heat generated when an electric current concentrates to a narrow breakdown spot immediately after the breakdown irreversibly forms a conductive spot having a relatively low resistance. If, however, the effect of the Joule heat immediately after the breakdown is insufficient, the resistance value rises with time to produce an unstable state, and this may finally make data unreadable and lost.
In the gate oxide e-fuse as described above, after dielectric breakdown of the gate oxide occurs, an electric current of about a few mA is supplied to the breakdown spot to obtain a hard breakdown state by the generated Joule heat. This is important to stabilize the characteristics after the breakdown. If the effect of the Joule heat is unsatisfactory, an incomplete breakdown state called soft breakdown occurs, and this may raise the resistance again to make data disappear.
To ensure the reliability, therefore, the characteristics are conventionally stabilized by obtaining a hard breakdown state by raising the current supply capability of a charge pump circuit which generates a high voltage for programming to about a few mA which is probably necessary after the breakdown. However, the charge pump circuit is normally a multistage circuit including four to five stages, so a large-capacity boosting capacitor is necessary to raise the current supply capability. As a consequence, the pattern occupied area of the program circuit, particularly, the charge pump circuit increases in the OTP memory, and this decreases the layout efficiency.